Sharpness is one important factor that determines image quality. An edge emphasis circuit is used to improve the sharpness. FIG. 4 is a circuit diagram for explaining a conventional edge emphasis circuit. Referring to FIG. 4, reference numeral 1001 denotes a video input terminal; 1002 and 1003, delay circuits connected in series to the video input terminal 1001 in the order named; 1004, 1005, and 1006, multipliers respectively connected to the video input terminal 1001, and the outputs of the delay circuits 1002 and 1003; 1007, an adder for adding the outputs from the multipliers 1004 to 1006; 1008, a multiplier connected to the output of the adder 1007; 1009, an adder for adding the outputs from the multiplier 1008 and delay circuit 1003; and 1010, a video output terminal connected to the output of the adder 1009. Also, reference numerals sl to s6 denote signals at respective circuits. FIG. 5 shows the waveforms of these signals.
In this arrangement, a video signal sl input from the video input terminal 1001 is input to the multiplier 1004 and first delay circuit 1002. The first delay circuit 1002 delays the input signal one pixel, and outputs a signal s2. The signal s2 is input to the multiplier 1005, second delay circuit 1003, and adder 1009. The signal s2 input to the second delay circuit 1003 is further delayed one pixel, and is output as a signal s3. The waveforms of these three signals sl, s2, and s3 are as shown in FIG. 5. That is, these signals are delayed one pixel each.
These signals sl, s2, and s3 are respectively input to the multipliers 1004, 1005, and 1006, and are respectively multiplied by 1/2, 1, and 1/2. The signs of the signals sl and s3 are inverted, and the inverted signals sl and s3, and non-inverted signal s2 are input to the adder 1007. The adder 1007 adds these signals, and outputs a sum signal s4. That is, the signal s4 assumes a value obtained by subtracting the average value of the previous and next image data from an image data value at a given point, i.e., a second derivative value at that point. The waveform of the signal s4 is as shown in FIG. 5.
Furthermore, the multiplier 1008 amplifies the signal s4 to obtain a signal s5. The signal s5 is an edge-emphasizing signal. The adder 1009 adds the signal s2 output from the delay circuit 1002 to the edge-emphasizing signal s5, thus generating an edge-emphasized video signal s6. In FIG. 5, the edge-emphasized signal s6 is indicated by the solid curve, and the original signal s2 is indicated by the broken curve to overlap the signal s6.
However, in the conventional edge emphasis circuit described above, the leading edge of an edge portion of the signal can be improved, while overshot and undershot portions are produced, as shown in FIG. 5. If such overshoot and undershoot are considerable, the image quality deteriorates due to a fringe formed at the boundary. When the coefficient of the multiplier 1008 is decreased to eliminate such overshoot and undershoot, the edge emphasis amount decreases in turn, and the improvement effect of the leading edge of the edge portion becomes small.
As described above, in the conventional edge emphasis circuit, the leading or trailing edge of the edge portion can be improved to obtain a sufficient edge emphasis effect, while considerable overshoot and undershoot result in deterioration of image quality.